1. Field of the Invention
This invention pertains to the fabrication of integrated circuits utilizing thin film layered superlattice material, and more particularly to processes for improving the microstructure of such thin films.
2. Statement of the Problem
Layered superlattice material thin films have been a very strong candidate for high density ferroelectric random access memories (FeRAMs). However, some problems in the layered superlattice material thin film have been pointed out, such as low remnant polarization (Pr), high crystallization temperature and surface roughness in comparison to Pb(Zr,Ti)O3 (PZT) thin films. Especially, the roughness of layered superlattice material thin films, in other words, high porosity of layered superlattice material thin films, causes some damage during layered superlattice material thin film patterning processes, leading to ferroelectric memories with poor reliability.
It is known that in the chemical solution deposition (CSD) process both electrical properties of layered superlattice material and crystallinity of these materials, including surface morphology, crystal orientation, etc., are affected by the baking process. During the baking (drying) process of as-coated wet thin films, which are usually carried out on a hot plate, metalorganics in wet thin film can be partially or fully decomposed. In the prior art, surface roughness improvement by adjusting these process parameters has always been obtained at the expense of polarizability, the key property for ferroelectricity. Thus, the choice was good microstructure of the integrated circuit or high polarizability.
Ultraviolet radiation has been known to be effective in assisting chemical reactions in forming thin films. For example, see Japanese Patent Application No. 61183921 of Kamei; Japanese Patent Publication No. 60-128264 (NEC Corp.); U.S. Pat. No. 4,811,684 issued to Tashiro et al.; U.S. Pat. No. 4,683,147 issued to Eguchi et al.; Japanese Patent Publication No. 62-22420; and A. Yamada, Y. Jia, M. Konagai, and K. Takahashi, “Photochemical Vapor Deposition of Si/Si1-xGex Strained Layer Superlattices at 250° C.”, Japanese Journal of Applied Physics, Vol. 27, No. 11, November 1988, pp. L2174-L2176. However, while UV has been found to improve the decomposition of the precursor and improve certain crystallization factors, it also has been found to decrease polarizability. There remains a need during the fabrication of integrated circuits to improve the exposure and benefits of UV radiation versus the contrary effect on polarization characteristics of the layered superlattice material.